Laminated structure

This application is a Continuation of International Patent Application No. PCT/JP2022/014563, filed on Mar. 25, 2022, which claims the benefit of priority to Japanese Patent Application No. 2021-059368, filed on Mar. 31, 2021, the entire contents of which are incorporated herein by reference.

One embodiment of the present invention relates to a laminated structure including a gallium nitride film formed over a substrate.

Gallium nitride (GaN) is characterized as a direct bandgap semiconductor with a large bandgap. This feature of gallium nitride is utilized, and a light emitting diode (LED) using gallium nitride has already been in practical use. Gallium nitride also has the characteristics of high electron saturation mobility and high breakdown voltage. In recent years, the characteristics of gallium nitride are utilized, and a transistor for a high-frequency power device has been developed. A gallium nitride film for a light emitting diode or a transistor is generally formed on a sapphire substrate at a high temperature of 800 degrees to 1000 degrees using MOCVD (Metal Organic Chemical Vapor Deposition) or HVPE (Hydride Vapor Phase Epitaxy).

In recent years, the development of a so-called micro LED display device or a mini-LED display device in which minute micro LEDs are mounted in pixels on a circuit substrate is proceeding as a next-generation display device. The micro LED display device or the mini LED display device has high efficiency, high brightness and high reliability. Such a micro LED display device or a mini-LED display device is manufactured by transferring a LED chip to a backplane on which a transistor using an oxide semiconductor or low-temperature polysilicon is formed (for example, see U.S. Pat. No. 8,791,474). Further, a method for forming a transistor and a light emitting diode including gallium nitride on the same substrate has been developed (for example, see U.S. Patent Application Publication No. 2020/0075664).

A laminated structure according to an embodiment of the present invention includes a substrate, and a gallium nitride film on the substrate. A surface arithmetic mean roughness (Ra) of a surface of the substrate is less than 0.33 nm.

A laminated structure according to an embodiment of the present invention includes a substrate, an alignment layer having a c-axis orientation or a (111) orientation of a face-centered cubic structure, over the substrate, and a gallium nitride film on the alignment film. A surface arithmetic mean roughness (Ra) of a surface of the substrate is less than 0.33 nm.

A laminated structure according to an embodiment of the present invention includes a substrate, a protective layer on the substrate, an alignment layer having a c-axis orientation or a (111) orientation of a face-centered cubic structure, over the protective layer, and a gallium nitride film on the alignment film. A surface arithmetic mean roughness (Ra) of a surface of the substrate is less than 0.33 nm.

In general, a gallium nitride film is deposited on a sapphire substrate at a high temperature. Since a large-area substrate such as an amorphous glass substrate has low heat resistance, a gallium nitride film formed at a low temperature has a problem of low crystallinity. Further, there is another problem that it is difficult to provide a gallium nitride film using a large-area backplane.

In view of the above problems, one object of an embodiment of the present invention is to provide a laminated structure in which a gallium nitride film with high crystallinity is formed over a substrate that can have a large area.

Hereinafter, each of the embodiments of the present invention are described with reference to the drawings. Each of the embodiments is merely an example, and a person skilled in the art could easily conceive of the invention by appropriately changing the embodiment while maintaining the gist of the invention, and such changes are naturally included in the scope of the invention. For the sake of clarity of the description, the drawings may be schematically represented with respect to the widths, thicknesses, shapes, and the like of the respective portions in comparison with actual embodiments. However, the illustrated shapes are merely examples and are not intended to limit the interpretation of the present invention.

In the present specification, the expressions “a includes A, B or C”, “a includes any of A, B and C”, and “a includes one selected from the group consisting of A, B and C” do not exclude the case where a includes a plurality of combinations of A to C unless otherwise specified. Further, these expressions do not exclude the case where a includes other elements.

In the present specification, although the phrase “above” or “above direction” or “below” or “below direction” is used for convenience of explanation, in principle, the direction from a substrate toward a structure is referred to as “above” or “above direction” with reference to a substrate in which the structure is formed. Conversely, the direction from the structure to the substrate is referred to as “below” or “below direction”. Therefore, in the expression of a structure over a substrate, one surface of the structure in the direction facing the substrate is the bottom surface of the structure and the other surface is the upper surface of the structure. In addition, the expression of a structure over a substrate only explains the vertical relationship between the substrate and the structure, and another member may be placed between the substrate and the structure. Furthermore, the terms “above” or “above direction” or “below” or “below direction” mean the order of stacked layers in the structure in which a plurality of layers are stacked, and may not be related to the position in which layers overlap in a plan view.

In the specification, terms such as “first”, “second”, or “third” attached to each configuration are convenient terms used to distinguish each configuration, and have no further meaning unless otherwise explained.

In the specification and the drawings, the same reference numerals may be used when multiple configurations are identical or similar in general, and reference numerals with a lower or upper case letter of the alphabet may be used when the multiple configurations are distinguished. Further, reference numerals with a hyphen and a natural number may be used when multiple portions of one configuration are distinguished.

The following embodiments can be combined with each other as long as there is no technical contradiction.

is a schematic diagram showing a configuration of a laminated structure according to an embodiment of the present invention. The laminated structureincludes a substrateand a gallium nitride film. The gallium nitride filmis formed on the substrateusing sputtering.

The substrateis a support substrate for the gallium nitride film. Since the gallium nitride filmof the laminated structureis formed by sputtering, the substratemay have heat resistance of about 600 degrees, for example. Therefore, for example, an amorphous glass substrate can be used as the substrate. Alternatively, a resin substrate such as a polyimide substrate, an acrylic substrate, a siloxane substrate, or a fluororesin substrate can be used as the substrate. In addition, the amorphous glass substrate or the resin substrate is a substrate that can have a large area. A polycrystalline substrate can also be used as the substrate. Since the polycrystalline substrate is larger than the sapphire substrate used in normal film deposition of gallium nitride films, the polycrystalline substrate can be used as a backplane in the same manner as the glass substrate or the resin substrate.

Crystal growth of the gallium nitride filmon the substratesuch as an amorphous glass is affected by the surface state of the substrate. In particular, unevenness on the surface of the substraterandomly cause the generation of crystal nuclei. As a result, since crystal growth of gallium nitride occurs in random directions, adjacent crystals interfere with each other and crystal growth is inhibited. Therefore, it is preferable that the substratehas a smooth surface with little unevenness. For example, the arithmetic mean roughness (Ra) of the surface of substrateis preferably less than 0.33 nm. Further, the root mean square roughness (Rq) of the surface of the substrateis preferably less than 0.39 nm. When the surface roughness of the substratesatisfies the above conditions, the crystallinity of the gallium nitride filmcan be improved even when the substrateis amorphous and is a glass substrate. In other words, the gallium nitride filmhaving a c-axis orientation (an orientation in which the surface has a (0001) plane) can be formed on the substrate.

Here, the deposition of the gallium nitride filmusing sputtering is described.

The substrateis placed to face the gallium nitride target in a vacuum chamber. It is preferable that the composition ratio of gallium nitride in the gallium nitride target is greater than or equal to 0.7 and less than or equal to 2 of gallium to nitrogen. Nitrogen can also be supplied to the vacuum chamber as a gas other than the sputtering gas (such as argon or krypton). In that case, it is preferable that the composition ratio of gallium nitride in the gallium nitride target is more gallium than nitrogen. For example, nitrogen can be supplied using a nitrogen radical source. The sputtering power supply source may be either a DC power supply source, an RF power supply source, or a pulsed DC power supply source.

The substratein the vacuum chamber may be heated. For example, the substratecan be heated at a temperature of greater than or equal to 400 degrees and less than 600 degrees. This substrate temperature can be applied to an amorphous glass substrate having low heat resistance. Further, this substrate temperature is lower than the deposition temperature in MOCVD or HYPE.

After the vacuum chamber is sufficiently evacuated, the sputtering gas is supplied to the vacuum chamber. Further, a voltage is applied between the substrateand the gallium nitride target at a predetermined pressure to generate plasma and the gallium nitride film is deposited. Since the gallium nitride filmis formed on the substratewhich has a controlled surface roughness, the gallium nitride filmhas a c-axis orientation.

The laminated structureaccording to this embodiment includes the gallium nitride filmwith high crystallinity and the c-axis orientation. Further, the laminated structureincludes the substratethat can have a large area. Therefore, by using the laminated structure, it is possible to increase the productivity of LEDs containing gallium nitride, or to manufacture a backplane on which transistors containing gallium nitride are formed.

is a schematic diagram showing a configuration of a laminated structure according to an embodiment of the present invention. The laminated structureincludes the substrate, an alignment layerand the gallium nitride film. The alignment layeris provided on the substrate. Further, the gallium nitride filmis formed on the alignment layerusing sputtering. In the following description, when the configuration of the laminated structureis the same as the configuration of the laminated structure, the description of the configuration of the laminated structure may be omitted.

The alignment layercan improve the crystallinity of the gallium nitride film. That is, the alignment layercan further improve the c-axis orientation of the gallium nitride film. Although the gallium nitride filmhaving a hexagonal close-packed structure is oriented along the c-axis so as to minimize surface energy even on the smooth substrate, the c-axis orientation of the gallium nitride filmcan be controlled by providing the alignment layeron the substrate. Specifically, the c-axis orientation of the gallium nitride filmis controlled by the type of material of the alignment layer. The alignment layercan use a material having a hexagonal close-packed structure, a face-centered cubic structure, or a structure equivalent thereto. Here, the structure equivalent to the hexagonal close-packed structure or the face-centered cubic structure includes a crystal structure in which the c-axis is not 90 degrees with respect to the a-axis and the b-axis. In the material having the hexagonal close-packed structure or the structure equivalent thereto, the alignment layercan have an orientation in the (0001) direction, that is, in the c-axis direction with respect to the substrate(hereinafter, it is referred to as the (0001) orientation of the hexagonal close-packed structure or the c-axis orientation.). In the material having the face-centered cubic structure or the structure equivalent thereto, the alignment layercan have an orientation in the (111) direction with respect to the substrate(hereinafter, it is referred to as the (111) orientation of the face-centered cubic structure). When the alignment layerhas the c-axis orientation or the (111) orientation of the face-centered cubic structure, the gallium nitride filmalso easily has the c-axis orientation. For example, titanium (Ti), titanium nitride (TiN), titanium oxide (TiO), graphene, zinc oxide (ZnO), magnesium diboride (MgB), aluminum (Al), aluminum nitride (AlN), aluminum oxide (AlO), silver (Ag), calcium (Ca), nickel (Ni), copper (Cu), strontium (Sr), rhodium (Rh), palladium (Pd), cerium (Ce), ytterbium (Yb), iridium (Ir), platinum (Pt), gold (Au), lead (Pb), actinium (Ac), thorium (Th), lithium niobate (LiNbO), BiLaTiO, SrFeO, SrFeO, BiFeO, BaFeO, ZnFeO, PMnN-PZT, or biogenic apatite (BAp) can be used for the alignment layer. In particular, it is preferable that titanium, graphene, zinc oxide, aluminum nitride, or aluminum oxide is used for the alignment layer. Further, the film thickness of the alignment layeris, for example, greater than or equal to 50 nm. In addition, the deposition of the alignment layeris not limited to sputtering. The alignment layercan be deposited using any method (apparatus) such as CVD.

As described above, the crystallinity of the gallium nitride filmis affected by the surface state of the surface to be deposited. That is, the gallium nitride filmis affected by the surface state of the alignment layerin the laminated structure. Therefore, the alignment layerpreferably has a smooth surface with little unevenness. For example, the surface arithmetic mean roughness (Ra) of the alignment layeris preferably less than 2.3 nm. Further, the root mean square roughness (Rq) of the surface of the alignment layeris preferably less than 2.9 nm. When the surface roughness of the alignment layeris under the above conditions, the gallium nitride filmhaving c-axis orientation can be formed.

In addition, since the alignment layeris formed on the substrate, the crystallinity of the alignment layeris affected by the surface roughness of the substrateto some extent. Therefore, also in the laminated structure, the substratepreferably has a smooth surface with little unevenness. For example, the arithmetic mean roughness (Ra) of the surface of substrateis preferably less than 0.33 nm. Further, the root mean square roughness (Rq) of the surface of the substrateis preferably less than 0.39 nm.

The laminated structureaccording to this embodiment includes the gallium nitride filmwith high crystallinity and the c-axis orientation. Further, the laminated structureincludes the substratethat can have a large area. Therefore, by using the laminated structure, it is possible to increase the productivity of LEDs containing gallium nitride, or to manufacture a backplane on which transistors containing gallium nitride are formed.

is a schematic diagram showing a configuration of a laminated structure according to an embodiment of the present invention. The laminated structureincludes the substrate, a protective layer, the alignment layerand the gallium nitride film. The protective layeris provided on the substrate. The alignment layeris provided on the protective layer. Further, the gallium nitride filmis formed on the alignment layerusing sputtering. In the following description, when the configuration of the laminated structureis the same as the configuration of the laminated structureor the laminated structure, the description of the configuration of the laminated structure may be omitted.

The protective layercan prevent the diffusion of impurities from the substrate. Therefore, since the alignment layerformed on the protective layerhas few impurities, the crystallinity is improved. For example, silicon oxide (SiO) or silicon nitride (SiN) can be used for the protective layer. In addition, the protective layermay be a single film or a laminated film. For example, when the substrateis an amorphous glass substrate, a laminated film of silicon nitride and silicon oxide (SiO/SiN) can be used. Further, when the substrateis a resin substrate, a laminated film in which silicon nitride is sandwiched between silicon oxides (SiO/SiN/SiO) can be used.

Since the alignment layeris formed on the protective layerIn the laminated structure, the crystallinity of the alignment layeris affected by the surface roughness of the protective layer. Therefore, the protective layerpreferably has a smooth surface with little unevenness in the laminated structure. For example, the surface arithmetic mean roughness (Ra) of the protective layeris preferably less than 0.51 nm. Further, the root mean square roughness (Rq) of the surface of the protective layeris preferably less than 0.60 nm.

The laminated structureaccording to this embodiment includes the gallium nitride filmwith high crystallinity and the c-axis orientation. Further, the laminated structureincludes the substratethat can have a large area. Therefore, by using the laminated structure, it is possible to increase the productivity of LEDs containing gallium nitride, or to manufacture a backplane on which transistors containing gallium nitride are formed.

In the following description, several modifications of the laminated structureaccording to this embodiment are described.

<Modification 1>

is a schematic diagram showing a configuration of a laminated structure according to an embodiment of the present invention. The laminated structureA includes the substrate, the protective layer, a first alignment layerA-, a second alignment layerA-, and the gallium nitride film. The first alignment layerA-is provided on the protective layer. The second alignment layerA-is provided on the first alignment layerA-. Further, the gallium nitride filmis provided on the second alignment layerA-. That is, the second alignment layerA-is in contact with the gallium nitride film.

The first alignment layerA-and the second alignment layerA-are different in the type of alignment layer. For example, the first alignment layerA-and the second alignment layerA-are an insulator and a conductor, respectively. Further, the first alignment layerA-and the second alignment layerA-may be a conductor and an insulator, respectively. The materials of the alignment layerdescribed above can be used in combination for each of the first alignment layerA-and the second alignment layerA-. The combination of materials for the first alignment layerA-and the second alignment layerA-may be appropriately selected in consideration of the device in which the laminated structureA is used. In addition, the second alignment layerA-with which the gallium nitride filmis in contact is preferably made of a material capable of preventing the diffusion of impurities from the first alignment layerA-to the gallium nitride film. Although the combination of materials of the first alignment layerA-and the second alignment layerA-is, for example, aluminum nitride and titanium, titanium nitride and titanium, or titanium oxide and titanium, the combination of materials of the first alignment layerA-and the second alignment layerA-is not limited thereto.

Each of the first alignment layerA-and the second alignment layerA-can also be a conductor, depending on the device in which the laminated structureA is used. Further, although the laminated structureA including two alignment layers is described in this modification, the laminated structureA may include three or more alignment layers.

<Modification 2>

is a schematic diagram showing a configuration of a laminated structure according to an embodiment of the present invention. The laminated structureB includes the substrate, the protective layer, an alignment layerB, and the gallium nitride film. The alignment layerB is provided on the protective layer. Further, the gallium nitride filmis provided on the alignment layerB.

The alignment layerB is patterned in a predetermined pattern. The patterning of the alignment layerB can be performed using, for example, photolithography. As shown in, the alignment layerB is patterned such that a portion of the protective layeris exposed. Therefore, the gallium nitride filmincludes a region A in contact with the alignment layerB and a region B in contact with the protective layerB. In the region A, since the gallium nitride filmis formed on the alignment layerB, crystal growth can be performed while the c-axis orientation is controlled. On the other hand, in the region B, since the gallium nitride filmis formed on the protective layer, the c-axis orientation is not controlled as compared with the region A. That is, the laminated structureB includes the region A with high crystallinity and the region B with low crystallinity. Therefore, in the laminated structureB, it is possible to form the gallium nitride filmincluding regions with different crystallinity by patterning the alignment layerB.

When the laminated structureB can be used for a backplane, it is possible to provide the backplane in which the crystallinity of the gallium nitride filmis controlled.

<Modification 3>

is a schematic diagram showing a configuration of a laminated structureC according to an embodiment of the present invention. The laminated structureC includes the substrate, the protective layer, a first alignment layerC-, a second alignment layerC-, and the gallium nitride film. The first alignment layerC-is provided on the protective layer. The second alignment layerC-is provided on the first alignment layerC-. Further, the gallium nitride filmis provided on the second alignment layerC-.

The second alignment layerC-is patterned in a predetermined pattern. The patterning of the second alignment layerC-can be performed, for example, using photolithography. As shown in, the second alignment layerC-is patterned to expose a portion of the first alignment layerC-. Therefore, the gallium nitride filmincludes a region C in contact with the first alignment layerC-and a region D in contact with the second alignment layerC-. Since the surface states of the surfaces to be deposited on which the gallium nitride filmis formed differs between the region C and the region D, the degree of crystal growth of the gallium nitride film(e.g., the ratio of the c-axis orientation (degree of orientation)) differs. Therefore, in the laminated structureC, it is possible to form the gallium nitride filmincluding regions with different crystallinity by patterning the second alignment layerC-.

When the laminated structureC can be used for a backplane, it is possible to provide the backplane in which the crystallinity of the gallium nitride filmis controlled.

<Modification 4>

is a schematic diagram showing a configuration of a laminated structure according to an embodiment of the present invention. The laminated structureD includes the substrate, the protective layer, a first alignment layerD-, a second alignment layerD-, and the gallium nitride film. The first alignment layerD-is provided on the protective layer. The second alignment layerD-is provided on the first alignment layerD-. Further, the gallium nitride filmis provided on the second alignment layerD-.

The first alignment layerD-is patterned into a predetermined pattern. The patterning of the first alignment layerD-can be performed, for example, using photolithography. As shown in, the first alignment layerD-is patterned such that a portion of the protective layeris exposed. However, the exposed part of the protective layeris covered with the second alignment layerD-together with the first alignment layerD-. That is, the second alignment layerD-includes a region formed on the first alignment layerD-and a region formed on the protective layer. Since each region has a different degree of crystal growth, the second alignment layerD-includes regions with different c-axis orientations or surface roughness. Further, the gallium nitride filmformed on such a second alignment layerD-also includes regions having different crystallinity corresponding to the respective regions of the second alignment layerD-. Therefore, in the laminated structureD, it is possible to form the gallium nitride filmincluding regions with different crystallinity by patterning the first alignment layerD-.

When the laminated structureD can be used for a backplane, it is possible to provide the backplane in which the crystallinity of the gallium nitride filmis controlled. Further, by using the uneven patterns of the first alignment layerD-and the second alignment layerD-as shown in, it is possible to control the light from the light emitting element and improve the light extraction efficiency.

is a schematic diagram showing a configuration of a laminated structure according to an embodiment of the present invention. The laminated structureincludes the substrate, the protective layer, the alignment layer, a non-alignment layer, and the gallium nitride film. The alignment layeris provided on the protective layer. The non-alignment layeris provided on the alignment layer. Further, the gallium nitride filmis formed on the non-alignment layerusing sputtering. In the following description, when the configuration of the laminated structureis the same as the configuration of the laminated structure, the laminated structure, and the laminated structure, the description of the configuration of the laminated structuremay be omitted.